The present invention generally relates to transformation of a material from a first state to a second state through the use of an applied energy beam. In one embodiment of the present invention, large grain polycrystalline silicon (p-Si) is formed from amorphous silicon by the superpositioned application of laser beams. Other embodiments of the present invention provide a method for manufacturing thin film transistors (TFTs) utilizing superpositioned laser annealing. Merely by way of example, the invention may be applied to the manufacture of TFTs for flat panel displays such as active matrix liquid crystal displays (AMLCDs), field emission displays (FEDs), and organic light emitting diode (O LED) displays. However, it would be recognized that methods in accordance with the present invention have a much broader range of applicability, including the formation of optical sensors.
Fabrication of high quality TFTs on transparent substrates is important for successful application of super-high-definition AMLCD technology. Excimer laser crystallization (ELC) is an efficient technology for obtaining high-performance p-Si TFTs for advanced flat panel display applications. In order to improve both the quality and uniformity of poly-Si TFT performance, the formation of high quality polycrystalline silicon films having carefully controlled grain size and location is often required. Pulsed laser crystallization of amorphous silicon (a-Si) filmsxe2x80x94usually effected by nanosecond, ultraviolet (UV) excimer laser radiationxe2x80x94has emerged as a promising new fabrication method. Laser annealing has been shown to be superior to other crystallization techniques because of its low fabrication cost and high efficiency. In addition, it is a low temperature processing technique because during the fast heating and cooling cycle the bulk substrate material is essentially unaffected, except within a submicron-thick thermal penetration zone adjacent to the heated film. This feature has important practical consequences, since it allows the use of inexpensive large area glass substrates, compared with more expensive quartz substrates capable of withstanding high temperature annealing.
ELC can produce grains of hundreds of nanometers in length depending on the a-Si film thickness and the sample preheating. However, the processing window is narrow because large grains can often only be obtained for laser pulse energy densities inducing near-complete melting. In the so-called superlateral growth regime, unmelted silicon particles in the vicinity of the film/substrate interface are thought to act as seeds for crystal growth in the lateral direction (i.e. parallel to the film surface).
Lateral crystal growth is important for improving electrical properties (e.g. electric field mobility) of the p-Si used by the TFT devices. Since the grains of p-Si are irregularly distributed, grain boundaries may deteriorate the electrical properties, reducing switching speed and increasing power consumption.
Therefore, a simple method and apparatus for easily transforming a layer of material from one state to another, for example from amorphous to polycrystalline, is desirable.
The present invention generally relates to transformation of a material from a first state to a second state, for example from amorphous to polycrystalline, through application of an energy beam. In one specific embodiment, large grain polycrystalline silicon (p-Si) is formed from amorphous silicon by the superpositioned application of laser beams. Embodiments of the present invention relate to large direction- and location-controlled p-Si grain growth utilizing recrystallization from superpositioned laser irradiation. The superpositioned laser irradiation controls cooling and solidification processes that determine the resulting crystal structure. Specifically, a first laser beam of a first pulse duration is used to melt an amorphous silicon (a-Si) film and to create a temperature gradient. After an initial delay, a second laser beam with shorter pulse duration is superpositioned with the first laser beam. When a-Si is irradiated by the second laser beam, the area heated by the first laser beam becomes completely molten. Spontaneous nucleation is initiated in the supercooled liquid-Si when the liquid-Si temperature drops below the nucleation temperature. However, the central part of the liquid pool that is subjected to continued heating by the first laser beam cools down slowly. Grains nucleated in the periphery of the fully molten spot can therefore grow into the liquid-Si and extend in length until they collide at the center of the first laser beam spot. The first laser beam prolongs the molten Si phase and induces grain growth in a certain direction. The second laser beam triggers nucleation and controls grain location leading to subsequent lateral grain growth.
One embodiment of a method in accordance with the present invention for fabricating a film of material comprises the steps of providing a layer of material, the layer of material being substantially of a first state and selected from a conductive material, a semiconductive material, or a dielectric material. A first energy beam is applied to the layer of material at a first time and for a first duration. A second energy beam is applied to the layer of material at a second time and for a second duration, the second time subsequent to the first time and the second duration expiring on or before the first duration, such that the layer of material is converted from the first state to a second state.
One embodiment of an apparatus for forming a film of material in accordance with the present invention comprises a first energy source emitting a first energy beam and a second energy source emitting a second energy beam. A first delivery element is configured to deliver the first beam to a position on an amorphous silicon film for a first duration; and a second delivery element is configured to deliver the second beam to the position after the first beam and for a second duration superpositioned within the first duration.
These and other embodiments of the present invention, as well as its advantages and features, are described in more detail in conjunction with the text below and attached figures.